Electrodeposition of selenium



Aug. 18, 1953 A. voN HIPPEL. ETAL 2,649,409

ELECTRODEPOSITION OF SELENIUM Filed July 30, 1943 5 Sheets-Sheet 1 INVENTORS ARTHUR l/o/v //PPa M. c'. 00M

Aug. 18, 1953 Filed July 30, 1945 3 Sheets-Sheet 3 AMPS/f7 2 ao as' @ggf | l l I EFFECT oF fic/o SM5/wm o 15D CUR/PEN? BEA/677' Y SOL uno/vs af- .S'e 02 /lv H2 604 AC/D 50u/770,11 SHPE/very KMOPMAL/ry) INVENTORS ,4N/fw? Vzw f//PPEL BY M. c. zoo/w Patented ug. 18, 195.3

ELECTRODEPOSITION F SELENIM Arthur een nippel, Weston, and Mortimer o. Bloejimyewton Highlands, Mass., assigner; by

assignments, to Federal yTelephone and `tion of ,Delaware Application July so, 1943, serial No..'21'96,'7`60 1 Claim. (o1.- eolie-ssa The present "invention `rel4 tes generally 'to 'the -electroedep osition oi Vv'selexiiu'rn nffcnduethg .surfaces, and more particularly `to the 'eletjdeposition of selenium in the grey ymetallic form.

This is a continuationinipart-four copending application Serial No. 420,906 filed November 29, 1941 (now abandoned) It is an object of .the inventiotnto .electrically deposit coatings o`f selenium of th'efgre'y crystalline metallic form.

It is a 4furthe'rfobi'ectfto.felectrieally ydeposit such coatings of metallic selenium which shall befsubstantially ir'ee :from amorphous fred selenium.

It is a f urther object to deposit such coatings which shall be adherent, malleamafductile, and resistant to abrasions'hock,an d distortion.

It is a Afurther Yobject lto depisfit `such l'coatings which shall be non-porous and vcorrosionresistant. A l y l It is a Vfurther object to deposit such coatings which shallbe capa-ble osfbingu'sed as elements of relectrical resistorsrectiers, photocells, etc.

In thepast, the-'former selenium which most frequently resulted from chemical reactions such as those which 1were employed in experiments-0n electro-deposition, was thevnon-metallic amorphous form known as ""i'ed amorphousseleniuh1. The grey metallic form of selenium, when desired, was usually produced from the red amorphous form thereof bysuitable heat treatment. Sofar as we are aware, nomethods have everb'een discovered of electro-depositing selenium invits metallic fform.

In accordance Withthe;y presenti-nvention, however, we have discovered-that selenium *may-'be electro-deposited in itsvge'y metallicform by einploying an electroe'deposition bath'containin'gan ample supply of quadrivalentjselnium cations, that is, cations containingfseleniumin the quadrivalent state. -Two veiainpl'es yaref Set-4 Vand seo+2.

Although there are several different pssible ways of producing-quadrivalent selenium cations, it appears that any bath/which Willgivefanadequate supply of these ions canbefada'ptedfor electroplating I grey :metallic selenium, provided either that such bath doe'snot at the'sametiine furnish a substantial supply o`fother positively charged ions Whose Critical depositionpotentlal in respect to the cathode material .and operating conditions is aslow as,o -lewertham that of the quadrivalent seleniumfcations, or that f any-such low-depositionepotential positive l ions which lmay be furnished are such that discharging they do not interfere with the deposition of grey selenium.

'bath solutions inwhich the desired supply of 'duad'ri'valent lselenium cations may be had resulting in the desired vdeposit of grey metallic selenium.

*"W'e have also 'discovered that even though the concentration of quadrivalent selenium cations `is not ordinarily suicient to produce a metallic deposition, the desired metallic deposition may nevertheless `be nhad by introducing other conditions. One such condition vis the use of ja high bath temperature. Thus,We have discovered that "an SeOa solution Without an excess of acid .may

be used provided the temperature of the solution is suihci'entlyjelevated for example, approaching C. or higher.

For practical purposes, this eXpedientis not as Satisfactory as the use ofy an excess of acid because ra greater amount of the deposit and Aat a much faster lrate can be had by use of the solution containing the excess of acid than When no excess of ac idvisused.V 'When an excess of acid is 'used a 4depositvmay be had at any temperature Within-awide range rof temperatures; for example, a deposit `'can be obtained at a loW rate by the u'sefof small current densities even at room temperature when thereisa suficient excess of acid,

although elevated temperatures within practical limits giove better and more practical results. A bath temperature of around 100 C. -has Vbeen found ktofgive very satisfactory results permitting a rapid deposit v/ith high efliciency, resulting in a uniform Yiine deposit of metallic selenium vfree from corrosion and pitting.

Weuhave '-fur'the'r discovered that metallic selenium can be deposited on cathodes of specific `this p 'urpse are selenidedcathodes rand tellurium 'cathodesi @ne method ofpreparing such cathodes has been disclosed in the copending appli- @spense-darne. 402,103, nied July 12', 1941, by Arthr von Hippel, upon which Patent No. 2,408,116 was issued September 24, y1946. This E patent discloses that a surface of nickel, copper, zinc, iron, tin, steels and magnesium-aluminum alloys is reacted upon by solution of selenium dioxide whereby a selenide coating is formed on the surface.

We have furthermore discovered that the metallic selenium electrodeposition is facilitated by use of an electrolyte which is capable of dissolving selenium; and we have found that such solvent solutions permit depositions even at low temperatures, such as room temperature, under conditions where ordinarily there would be insufficient quadrivalent selenium cations or insufhciently high temperature to produce the metallic deposition. Apparently the presence of the solvent action is needed at temperatures below quantities in an aqueous bath which also coni tains an acid, e. g., I-IzSOi. The positive hydrogen ions from the acid tend to favor the production of quadrivalent selenium cations by combining with the oxygen'of theSeOz compound and forming water. We have found that such acids as sulphuric acid (HzSOi), selenic acid (H2Se04), nitric acid (I-INOa), and hydrochloric acid;(HCl) are Yespecially suitable for enhancing the concentration lof quadrivalent selenium cations. For simplicity it may be convenient to consider atheoretcal explanation of the reaction 50 C. even ywhen there is a preponderancerof Y,

quadrivalent selenium cations. The solutions can be made to act as solvents by the use of hydrochloric or sulphuric or perhaps nitric acid.

In the drawings, Fig. l represents the relation between temperature and rate at which nuclei of metallic selenium are formed. Fig. 2 depicts the variation of cathode eiiiciency as a function of SeO2 concentration. Fig. 3 shows the effect of temperature on permissible current density. Figures 4 and 5 are graphs of permissible current density versus acid strength. For Fig. 4 the content of SeOz was xed at 840 grams per liter of solution, whereas for Fig. 5 the acid solutions were saturated with SeOz.

In practicing our invention we have found that the factors of SeOz concentration, acid concentration, temperature and current density should be kept within proper limits and be maintained mutually consistent with each other to produce the best quality of metallic deposit. These factors operate as follows:

(a) Increasing the SeOz concentration increases the tendency toward metallic selenium deposit.

(b) Increasing the acid concentration increases the tendency toward metallic selenium deposit except, however, solutions which are too strongly acid should not be used at too high a temperature, otherwise they may attack the material being plated; and furthermore, too much acid at too low a temperature should not be used, otherwise it may make the SeOz precipitate out.

(c) Increasing the temperature of the bath increases the tendency toward metallic selenium deposit, provided, however, that the temperature must not be raised too high if a fine grain deposit is desired. For example, a temperature above about 120 C. tends to create a coarse, rough grain. Furthermore, a temperature very much in excess of this may tend toward excessively slow gray crystal formation, leaving a type of red selenium deposit which may not disappear until plating has taken place for an excessive time.

(d) Decreasing the current density tends to produce the metallic type of selenium deposit; and when the current exceeds a certain amount for any given set of the other three conditions there is a tendency toward the red amorphous deposit. The effect of the foregoing factors on the electrodeposition on the metallic selenium can be observed from the following examples of variations in these factors.

The preferred method of producing a bath adequate for the electro-plating of metallic selenium A to be as follows:`

Actually, since the reaction occurs in an aqueous solution, the SeOz should probably be considered as forming selenious acid with the water of the bath, and therefore the reaction might more properly be written as follows:

The above might also be written as a single twostep reaction, thus:

In these latter equations the negative radical of the acid is omitted, since only the hydrogen ion of the acid is effective in increasing the concentration of quadrivalent selenium cations. The concentration of the quadrivalent selenium cations is sufficient for the purpose when nearly all the cathode current is carried by these cations.

In order to obtain a deposit of grey metallic selenium using an aqueous electrolyte, it is not enough to insure the presence of the quadrivalent selenium cations, but proper operating conditions must be maintained as well. Thus, at low temperatures it is necessary to limit the applied voltage to such values that the resulting current can readily be carried by the available quadrivalent selenium cations. Under these conditions a high cathode efficiency is obtained and the evolution of hydrogen at the cathode is avoided or reduced to a harmlessly low value. It has been `found that when an excessive amount of hydrogen gas is evolved-by passing a current too great to be readily carried by the quadrivalent selenium cations-the selenium deposit produced is of the amorphousred form rather than the desired grey metallic form. It is believed that the reason for this .is that the high concentration of nascent hydrogen atoms in the solution at the cathode tends to produce from the SeOz, Se atoms which form amorphous aggregates. The spongy nature of the amorphous deposit supports this assumption. The current density atwhich this phenomenon becomes pronounced is much less at lower temperatures than at elevated temperatures. In accordance with our invention, therefore, it is proposed not only to select the ingredients of the bath as outlined above, so as to produce a supply of the quadrivalent selenium cations, but also to regulate the current density to such a value that nearly all the cathode current is carried by these cations.

A further factor of major importance must be taken into consideration for the prescription of proper operating conditions. In the general field asias@ of crystal growth phenomena, and partlcularlylll the field ofthe generation, of different polymorphous forms, it has been Well established that the particular crystalline form, as Well as the size of the crystals generated, is, profoundly influenced by the temperature. Crystal nllCli producing the crystal type desired, form. only in a limited temperature range, and the growth velocity of such nuclei into. a more .0 1 1135.5. m16 crystalline deposit is another function ofthe. tem: perature. We have established that in the, case of selenium, the generation of the grey mfldllic variety from the ordinary amorphous form by nuclei formation has a temperature characteristic substantially that given by Eig. l. The rate of formation of nuclei ofthe. grey `forni pa ses through a maximum in the neighborhood o fv 49,0," to 100" C., and drops steeply on either side of this interval to lW values. If the. bath iS.. Opz-lated at temperatures at which the. transformation velocity from the red into. the grey form very low, any red material. generated stays as such for a long time. If, on the other. hand, the bath temperature is kept at a level corresponding to rapid nuclei formation, any selenium. discharged in the red modification transforms so rapidly into the metallic selenium that interference With the plating process by this modi-lcation is avoided. For instance, it is possible t9. elllplate grey selenium in the preferred temperature range indicated by Fig. l, at such` high current densities that substantial amounts of hydrogen are evolved-a procedure, leading, at` lowl temperature, e. g., room temperature, to the formation of disturbing quantities of red selenium.

Apart from the temperature. factor, there is the further factor of importance, namely the solubility of selenium in the bath. If the `bath be active as a solvent, red selenium which might deposit at the cathode will dissolve andl redeposit as grey selenium, Bathsy foundsuitlble as solvents are: (l) concentrated HC1 containing about to 50% by weight. of SeQz and (2) 18% by Weight. of -SeOzr in H2S.Ol the strength of which varies from about `50% to 90% byl volume.

As an example of theimportance. of theseconditions of high temperature and O SQlllbill'ly, the case of a SeOz bath withoutexcess of acid may be considered. Metallic. deposition cannot be had in it at temperatures-below about 50 C. as selenium is not soluble` in it. Butiffthe temperature be raised wellaboye 50?. C., such as around 100 C., this higher tem... rature converts the nlm of red deposit initially-formed to metallic grey form; and subsequent Plating is. in the Inetallic form. If on the other halldi the. Solubility of selenium in the solution were increased as by the HCl or H2504 noted above, the red lm which would be formed, would also be converted .tothe grey even belOW .50i-1C.

We have furthermore establishedthatonce a deposit of the grey form has been obtained, it is subsequently possible to continue plating outside of the preferred temperature range, because the already existing greymaterial forcesthevrlcomingr new atoms into,its, .col llgllration. Therefore, in this situationplating. is possibleI under operating conditions which., wouldn produce red selenium if originally applied to the bare cathode. surface. Consequently it isadvantaefllls t0 start plating with a low current density, and to step. up the Current attera., Suilelll @Vetage by grey selenium has been obtained. It` will thus be seen that according. Vto our invention notv only the choice of the composndsL dissolved in the bath but also the adjustment of the concentration, the current densities. and the temperatures and solubility are important for attaining the desired result ofdepositing grey metallic selenium free from the formation of substantial quantities of red `amorphous selenium.

Ii not only a deposit ofl selenium in the grey modification as such is desired, ,b,u.t also. its dep,- ositlon in a smooth layer 'Without pores, Vadditional precautions are necessary. Gas evolved from the cathode, or migrating towards, it, has the tendency to form vtena-ciously adhering bubbles on the cathode surface, especially in vis,- cous electrolytes. At these spots Where` the bubbles adhere, `deposition of selenium Ais prevented and pits are left in the final deposit. Reduction of the viscosity ofthe solution by increasing the temperature, or by decreasing the concentration, or by removal of the bubbles by agitation is necessary if the interfacial tension cannot be adequately. loweredv by wetting agents.

In order to illustrate as specifically as` possible the effect of the variables mentioned aboye on the character .of the deposit, We give the following quantitative examples:

A bath composed of selenium dioxide sulphuric acid solution, containing` 13 normal HQSOl at a temperature ofa55"v C., will be chosen as the first example. Tov nndV the proper SeQz Concentration. we measure the cathode eifency (i. e., the percentage of the current which, is carried by quadrivalent selenium cations) of the bath as a function of selenium dioxidecontent (Fig. 2). Reasonably satisfactory .results for some purposesA can be obtained even when the current density is. so, high that, only about 6.5 percent of the cathode current is carried-by the desired euadrivalent selenium cations. It is preferred. however, t0 Operateunder conditions where at least percent (and preferably more than per cent) ofthe cathode currentis carried by these ions. ,As this characteristic curve indicates, at slightly over 800 g, of SeOzper liter of l normal sulphuric acid, 10D-percent enicielly is reached for current densities of `about 8 amp/sc. ft. Hence a solution of 18 normal sulphuric acid, containing 851;() gf. of SeOa per liter and operated atonly 55," C. with a current density of 3, arno/sq, ft. would carry the full current by quadrivalent selenium cations and give a deposit of metallic Se at thev cathode Without evolution of '-hydrogen. But, due. to the high viscosity and surface. tensionV of the solution, bubbles from o ccluded gas or from gas diffusing from the anode may adhere tenaciously to the cathode, pro ducin g pits, in the deposit. .an increase in bath.- temperature is advisable. to lower the viscosity,- andalso to. allow plating with higher current densities. It will be Anoted from Figure 2 thatthecathodeeiciency rapidly dro-ps off when less than g. of SeOz per litre of solution is used.

Fig. 3 illustrates the effect of tempera-ture on the maximum plating currentfpermissible for a bath containing segg/liter SeOzand the. acid strength. of 1 8` ,i xrrnal H2804 used. above. The effects limiting further raisingof thecurrent in the different temperature ranges are indicated. At the stillloiver25 C5, a. limiting factor is the formationv of a deposit Aof ordinary red amorphousV selenium which occurs` even with current densitie below 1.arno/str1..v tt. A1255? C. audities fate?, uridine, bubble-.formation as; already; minted. 0st are.. el 1;0Qt Q. the

current density can be raised to 75 amp/sq. ft. before the bubble phenomenon interferes. At 125 C., 230 amp/sq. ft. can be used without signicant bubble evolution, but at this temperature and to a much more pronounced degree at 150 C., the decreasing rate of nuclei formation interferes (see Fig. 1). Any slightest film of red selenium which may form tends to persist at these temperatures. The conversion from amorphous red selenium t the grey crystalline form is reasonably rapid once a nucleus is started, but such nuclei are slow to form. The result is that comparatively few nuclei are formed, and then the conversion to the grey form centers on these few nuclei. This produces a rough deposit of relatively large metallic crystals in circular bunches. Preferential deposition takes place on these clusters, further roughening the structure.

If this effect is not objectionable (e. g., if the electro-deposit is effected solely for the purpose of purifying the selenium, or for some other purpose where an irregular grain structure is unimportant) the temperature region above 120 C. may be employed. If an even grain is desired, however, the higher temperatures are not recommended for plating on the bare cathode because an extremely great reduction of current density would be required to give a fine even grain at such temperatures. If a cathode is used covered already with a smooth deposit of grey selenium, this limitation will not exist. Consequently the temperature region around 100 C. is most appropriate for achieving a smooth deposit with reasonable speed upon a bare cathode, although otherwise the temperature region above 120 C. may be employed as above mentioned.

Now, using a bare cathode in a sulfuric acid bath and keeping the temperature at 100 C. and keeping the SeO2 concentration at 840 g./liter at 100 C., the acid strength of the bath may be varied. Fig. 4 shows the highest permissible plating current as a function of acid strength. Without acid, considerable evolution of hydrogen takes place even at current densities below 1 amp/sq. ft. on account of the low quadrivalent selenium cation concentration; the deposit, therefore, becomes pitted. At 9 normal H2SO4 the same limitation appears at about 10 amp/sq. ft.; at 18 normal, as already mentioned, the current density may be raised'to 75 amp/sq. ft. before this phenomenon becomes serious. Further increase in acid strength brings the solution into the region where the 840 grams of SeOz per liter of acid are not completely soluble (solubility limit).

Assuming that 18 normal H2SO4 at 100 C. thus has been established as a favorable condition, an increase in SeOz concentration may promise even higher current densities without hydrogen evolution. Fig. shows a situation analogous to Fig. 4, but the solutions are saturated with SeOz. As anticipated, much higher current densities can be carried before hydrogen bubbles pit the deposit; now at 18 normal H2804, more than 200 amp/sq. ft. can be used. At full acid concentration (36 normal H2SO4) a chemical reaction takes place at the cathode surface and no deposit is obtained.

The facts stated so far for sulphuric acid solutions only, are substantially the same for selenic acid solutions. In hydrochloric acid, favorable conditions have also been found. The chemical attack described above for concentrated sulphuric acid is not encountered with hydrochloric acid, and its viscosity is lower. The 'normally objectionable volatility of hydrochloric acid disappears if sufficient quantities of SeOz are dissolved therein. Nitric acid, while having Vthe good properties of hydrochloric acid as far as viscosity and hydrogen ion concentration goes, has the disadvantageous reactivity of sulphuric acid in high concentrations.

Excellent deposits can be obtained on platinum, nickel, silver, antimony, also steel and other metals.

In the case where the metal used for the cathode is one which is readily attacked by the acid used in the bath, however, certain special precautions may be necessary. One very suitable procedure is to connect up the current source in reverse polarity before immersing the cathode Thus, when this cathode is first immersed it is actually an anode and the positive hydrogen ions are kept away from the metal of the cathode A thin coating of a selenide, selenate (or possibly some other compound) of the cathode metal appears to be thus formed. Now the current is reconnected in its regular polarity so that the cathode is negative and plating commences. The thin protective coating appears to have no substantial deleterious effects but does protect the metal from the action of the positive hydrogen ions which might attack it at the commencement of plating, with consequent prevention of pitting or red amorphous selenium deposition. The formation of red selenium is thus prevented by a seleniding of the cathode. For instance, a bath containing selenic acid for the above-described brief reverse current treatment of the cathode may be used even if some other bath is to be used for the actual plating, since the formation of a selenide or selenate (or other compound) which results from such treatment is particularly advantageous as a preliminary to the plating of selenium upon the cathode.

Where it is possible to do so, it is preferred to choose the acid to be employed with respect to the metal employed as a cathode so as to prevent attack of the cathode by the acid. Where it is necessary or desirable from the standpoint of speed or economy to employ, as the cathode, a metal which tends to react with the acid used in the bath, special precautions must be taken in initiating the plating in order to prevent the acid from acting on the cathode before the selenium plating is started. In some cases such adverse action can be substantially avoided by arranging that the current shall be applied as soon as the cathode is inserted into the bath, e. g., by connecting up the wires andturning on the current source before immersing the cathode.

The electrodepositions made in accordance with our invention are useful either for the purification of selenium or in the making of rectiers or photocells or resistors for corrosion protection. For use as rectiers or photocells it is desired that the crystalline structure be fine and built up to a uniform thickness. A rectifier will ordinarily require a thicker deposit than a photocell. For the rectifier and photocell use, if a base cathode is used, it will ordinarily be desired to deposit the selenium at a temperature below 120 C. and preferably around C., in order to avoid the coarse porous type of deposit. If, however, the cathode is selenided before or upon immersion in the electrolyte, the electrodeposition results in a formation of gray selenium, the subsequent deposit will continue to be gray, as previously stated, and there will be no coarse grain due to the absence of the red selenium, even though the temperature is considerably above or below these values. Purification will arise from the fact that impurities in the selenium used for the deposit either will not deposit on a cathode at all or else have a threshold Value of deposit above that at which the selenium is being deposited on the cathode; so that these impurities are left behind and are not deposited. For purification purposes it will usually not be so important Whether the deposit is coarse or fine and the temperature at which the bath is operated can accordingly be given a greater latitude than When a uniform negrained deposit is desired, for example a temperature Within the limits of about 50 C. or 75 C. to about 145 C. or 160 C. might be used; and higher temperatures than 100 C. might in some cases be preferable owing to the greater permissible current density at which metallic selenium can be deposited free from red selenium at the higher temperatures. For example, a temperature of 120 C, or more may be desirable. For most pur` poses, however, around 100 C. is probably best, as this is the temperature range at which the crystal nuclei tend to form most rapidly. Selenium deposited for purification purposes can be scraped off 'the cathode and used elsewhere as desired.

What is claimed is:

The process for electrodepositing gray crystalline metallic selenium upon a surface of an article having a metallic surface from the group con- 19 sisting of nickel, silver, antimony and steel, that comprises producing an initial coating of a selenide of said surface metal thereon, and then making the article the cathode in an electrolyte consisting o an aqueous solution of sulphuric acid having a concentration of about 1S normal, and having between 15G grams of selenium dioxide per litre and sufficient selenium dioxide to form a saturated solution dissolved therein, maintaining he temperature of said electrolyte between C. and C., and maintaining the current density between 8 amperes per square foot and 230 am peres per square foot.

A. VON HIPPEL.

BJIO'RTIMER C. BLOG-M.

References Cited in the ile of this patent UNITED STATES PATENTS Number Name Date 1,743,169 Presser Jan. 14, 1930 2,197,497 Geisler et al. Apr. 16, 1940 2,202,532 Malcolm May 28, 1940 2,266,922 Thompson et al. Dec 23, 1941 2,408,116 Von Hippel Sept. 24, 1946 FOREIGN PATENTS Number Country Date 525,664 Germany May 27, 1931 OTHER REFERENCES La Ricerca Scientica, vol. 10 1939), pages 1143 and 1144; article by Sella. 

